Essential Coding Interview Questions Java Edition

Write a function that counts vowels a i o u in a string, case-insensitive, by iterating characters, converting to lowercase, and incrementing a counter.

Implement a function that takes an ordered sequence of numbers and returns true if a duplicate exists, otherwise false, by comparing each element to its successor.

Learn to implement a Java leap year checker using the rules: divisible by 4, not by 100 unless by 400, with tests for 1900s, 2020, and negative years.

Implement a bubble sort on an array of integers by swapping out-of-order pairs until no swaps occur, then return the sorted array as a string using a swapped flag.

Implement insertion sort to sort an array of integers and return the sorted array as a string, using a key and shifting larger elements one position at a time.

Learn how to determine if two numbers in an array sum to a target using a hash map, by checking complements and returning true or false.

Develop a Java solution to the ransom note problem by comparing word frequencies from newspaper and note, ensuring case sensitivity and using whole words with a frequency map.

Implement a power function for x and y that halves y with floor, returning 1 when y <= 0 and squaring or multiplying by x based on y mod 2.

Implement a method that takes a matrix of integers and returns its elements in a clockwise spiral using start and end indices for rows and columns.

Count islands in a matrix of ones and zeros using flood fill to group connected ones. Traverse the grid, explore four-direction neighbors, and increment per island with a stack.

Learn to compute the length of the longest common subsequence for two sequences. Explore how subsequences preserve relative order, not contiguity, using a bottom-up matrix and example cases.